This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present invention. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
The storage of large quantities of liquefied natural gas (LNG) at ambient pressure poses many technical problems. Of particular concern are the thermal loads and deflections imposed by the large temperature difference (˜180 deg C.) between a tank filled with LNG and an empty tank at ambient temperature. To mitigate the risk of structural failure or leaks, a high quality of fabrication is required resulting in high costs. For marine applications such as LNG tanks in ships or offshore facilities, additional problems are introduced due to dynamic loads and the deflection of the vessel due to waves.
Various designs have been developed which attempt to address these problems as well as other issues related to LNG containment. The most popular designs for shipboard applications are the membrane LNG tank and the spherical Moss tank. The membrane ship employs several tight layers of insulation on the inside of the hull's structure to protect the hull structure from the cold temperatures of the cargo. The Moss ship uses several large spheres which are supported at their equator by a skirt which isolates the cold temperatures of the cargo from the steel hull.
However, both membrane ships and Moss ships are labor intensive to construct. Membrane ships may be less expensive to construct than the Moss ships but are more susceptible to damage due to internal loads from sloshing cargo. The tanks of the Moss ship extend above the main deck and leave very little deck area on which equipment can be fitted. The lack of deck space afforded by the Moss design is of particular concern for offshore facilities where multiple large pieces of equipment are required to be fitted on-deck.
Both of these containment systems employ materials which are not typically handled by normal shipyards. Both designs require complex fabrication methods and a significant investment in facilities to enable the construction of these ships. Due to this large initial investment, only a handful of shipyards are currently able to construct LNG ships.
Another cargo containment system for marine applications is the self-supporting prismatic type B (SPB) tank disclosed in at least U.S. Pat. Nos. 5,531,178 and 5,375,547. The SPB tank is a prismatic aluminum, 9% Ni, or stainless steel tank which is free standing and rests on the inner bottom of a vessel's hull. The bulkheads, tank top, and bottom of the tank are fabricated with a traditional grillage of stiffeners and girders. The tank is supported by an array of steel & wooden chocks and is provided with external insulation to protect the hull from the cold temperatures of the cargo.
However, this system is considerably more expensive to build than membrane or Moss ships. This system is costly because the materials needed to handle the cold temperatures, aluminum, 9% Ni, or stainless steel, cannot be handled by magnets and are thus not able to be fabricated using much of the automated machinery used by shipyards in their normal construction. This results in a very labor-intensive manual fabrication process which is costly and prone to quality problems.
Reference is also made to U.S. Pat. No. 3,721,362 “Double Wall Corrugated LNG Tank.” This design employs independent prismatic tanks with bulkheads and decks comprised of a sandwich of two corrugated plates supported by a grillage of girders. The corrugations of the “Double Wall” design are longitudinal and the joining of the double plating would require significant welding and result in a void space which would be very difficult to inspect.
Accordingly, the need exists for an improved liquid-tight tank capable of withstanding sloshing loads, expansion/contraction loads, and external loads, and is relatively easy to manufacture.